Since large topological energy gaps have been observed in the magnon bands of CrI\(_3\)  and CrBr\(_3\)  [1]}, [2]}, [3]}, it is important to check whether or not such gaps appear also at the Dirac points of CrCl\(_3\) . Although the excitation spectrum in Fig. REF (a) is powder-averaged, its Dirac point at \(|\mathbf {Q}_{\frac{1}{3}\frac{1}{3}0}|\)  = 0.70 Å\(^{-1}\)  is clearly visible at \(\hbar \omega \)  = 4.4 meV. The apparent intensity discontinuity between the upper and lower modes is observed at the wave vector corresponding to the Dirac crossings of the linear dispersions. To check for the possible of a gap opening, we plot a few selected constant-\(Q\)  cuts of the inelastic neutron scattering intensities including the Dirac wave vector in Fig. REF  . The calculated intensities, which are convoluted by the instrumental resolutions only, are plotted together as solid lines. The excellent agreement between the observations and calculations indicates that there is no additional broadening in the excitation. Such behavior is in remarkable contrast to the excitation spectra of CrI\(_3\)  or CrBr\(_3\) , which exhibit significant broadening in energy throughout their magnon bands at low temperatures[1]}, [2]}, [3]}. The magnon broadening in insulating ferromagnets typically occurs via magnon-magnon interactions or magnon-phonon coupling as the temperature approaches \(T_\mathrm {C}\)  [7]}, [8]}, [9]}. We thus stress that the resolution-limited magnon in CrCl\(_3\)  at \(T =\)  4 K is considered normal whereas the magnon broadening observed in CrI\(_3\)  and CrBr\(_3\)  are anomalous. Since the magnon broadening in these van der Waals ferromagnets is beyond the scope of the current work, this subject will be investigated in a separate work. Taking this into account, we find that the constant energy cut at \(|\mathbf {Q}_{\frac{1}{3}\frac{1}{3}0}|\)  = 0.70 Å\(^{-1}\)  is well explained by a calculation without a gap opening contrary to CrI\(_3\)  and CrBr\(_3\) . The absence of the Dirac gap in CrCl\(_3\)  is consistent with its spins being oriented parallel to the honeycomb plane in zero field. Note that the gap openings in CrI\(_3\)  and CrBr\(_3\)  are ascribed to the Dzyaloshinkii-Moriya vectors oriented perpendicular to the honeycomb planes and parallel to the ordered spins [1]}, [3]}.
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